This disclosure relates to methods of forming silicon carbide fibers. More particularly, this invention relates to forming silicon carbide fibers from infusible polysilazane fibers by utilizing one continuous heating step.
High-strength, fine diameter silicon carbide (SiC) fibers are needed for ceramic matrix composites (CMCs). Polymer-derived ceramics are spun into fibers and heat treated to achieve the strength necessary for use in various applications. Currently, precursor ceramic polymer fiber wound on a spool is run through a pyrolysis process with two heating stages to another spool. One such method for polycarbosilanes is described in U.S. Pat. No. 5,928,978. In the first step, a precursor ceramic polymer fiber is heated at a temperature sufficient to convert the polymer to glass (for instance, about 900° C. to 1200° C.). This temperature is then held to fully convert the polymer into the glass. In a second heating step, the glass is heated to a temperature sufficient to convert it to crystalline ceramic (for instance, greater than 1800° C.). At least one of the heating steps is typically at a temperature high enough to allow outgassing to occur. If any step in this type of process is completed too quickly, if the ramp rate is too high, or if there is not enough hold time, various undesirable properties may be found in the resulting ceramic fiber, such as fracture of the fiber, shrinkage, or lack of removal of impurities. Conversely, if the fibers are in the hot zone for too long, or if the temperature is too high, the crystalline fiber may develop large grains which reduce the tensile strength. In another exemplary two-step process, Hi-Nicalon fibers are made by converting precursor polycarbosilane fibers into silicon carbide fibers by heating at 200° C. in air to cross-link the structures, then forming ceramic fibers by heating to 1200° C. in an inert atmosphere. In another process, a near stoichiometric polyaluminocarbosilane fiber has been developed which is cured by oxidation and utilizes a two stage pyrolysis, the first to 1300° C. (for formation of oxygen-rich silicon carbide fiber), and the second to 1800° C. (for the outgassing of CO and sintering).
In order to accommodate a process with two required, disparate temperature conditions, the current two-step heating processes require either two separate furnaces or a larger furnace that allows for the presence of two different heating zones. A single-stage heating process would provide beneficial process simplicity, including shorter treatment times and lower capital investment in furnace equipment.
Therefore, there is a need for a process for forming silicon carbide fibers utilizing a single heating stage.